There are many well known variable speed drive systems for AC machines commonly in use.
Controllers for these systems consist of two basic types, that being either line commutated or force commutated.
Force commutated type controllers include (transistorised or GTO) adjustable voltage or current inverters, or pulse width modulation inverters. In all cases, there is a requirement that the available mains frequency be DC converted by a rectifier circuit, then inverted to the required AC frequency for provision to the AC machine. A characteristic of such controllers is the expense associated with the extra elements in the DC conversion process, including the necessary energy storage facility in the DC link.
Line commutated controllers, which utilise SCR technology, rely on a zero crossing of the line current in any phase, together with the impression of a reverse bias voltage between the anode and cathode of the respective SCR, and are commonly provided by means of a rectifier/inverter combination (this requiring provision of a DC link), or through a direct AC/AC converter such as a cycloconverter.
A characteristic function of a cycloconverter is that it produces an output frequency of some submultiple of the supplied input frequency (typically the mains frequency).
Cycloconverters are limited by an upper frequency bound, which is a function of the number of SCRs provided and their configuration. For a three-phase, three-pulse cycloconverter having a 50 Hz mains input frequency, the maximum output frequency to be generated will be approximately 20 Hz. The upper bound of output frequency from a cycloconverter results from an increase in harmonic components in the generated waveform, which will cause a dramatic reduction in the quality of that waveform and thereby increase the degree of heating within the AC machine due to losses. A further consequence of the deterioration of the output waveform is that unacceptable torque pulsations may occur within the AC machine.
All the foregoing control systems can provide variable speed control of an AC machine within a defined range. However, those systems which provide for speed control over the range of 0 to mains frequency tend to be particularly expensive. Accordingly, there is a need to provide a variable speed AC control system which is relatively inexpensive and can provide speed control over the full range.
The present invention attempts to alleviate some of the problems associated with the known systems by providing a system which is relatively simple in design and can be operated by a method which provides speed control over a full range.